1. Field of the Invention
The present invention relates to a bone assessment apparatus and bone assessing method, and more particularly to an apparatus and method for assessing a bone based on attenuation factors of an ultrasonic wave which has passed through the bone.
2. Description of the Related Art
Due to the rapid increase in the number of elderly people in the population, bone disorders such as osteoporosis and osteomalacia are becoming more common, and a great need has emerged for better diagnosis and prophylaxis.
Among bones, especially spongiosa (i.e. cancellous bone) consists of trabecula which is relatively hard and bone marrow which is relatively soft. In other words, marrow fills the spaces in the trabecula extending lengthwise and crosswise to form the spongiosa.
Bone diseases cause the bone structure to be weakened. The density of trabecula of those who are in good health is relatively high, while the density of trabecula of those who are suffering from a bone disease such as osteoporosis is relatively low.
When an ultrasonic wave is irradiated to a bone, the attenuation factor of the transmitted ultrasonic wave varies depending on the frequency. The attenuation characteristic is also varied by changes in the bone structure. When irradiating an ultrasonic wave to a bone where the trabecula is coarse, the attenuation factor of the ultrasonic wave becomes smaller.
An apparatus for diagnosing bone disease by detecting and observing the attenuation characteristics of the ultrasonic wave has been proposed in, for example, U.S. Pat. No. 4,774,959. In such an apparatus, two different frequencies are fixedly set, and an attenuation factor is obtained at each of the predetermined frequencies. Then, the gradient of the attenuation factor relative to the frequency is computed based on the difference between the two attenuation factors. However, in such a conventional apparatus, differences among individuals are not considered, and therefore, reliable bone assessment results can not be obtained.